Most Unwanted: Japanese knotweed
Fallopia japonica
Habitat: Terrestrial
Threat: Threatens protected species and habitats
Status: Established
This species was introduced into Irish gardens probably in the middle of the nineteenth century. Only female plants are present in both Ireland and Britain and as such this plant is currently incapable of producing seed here. It is nonetheless highly invasive and ecologically damaging.
This species has been nominated as among 100 of the "World's Worst" invaders
What is it
This is a relatively large plant that can grow up to 2 – 3 m in height and can dominate an area to the exclusion of most other plants. It can form an extensive network of rhizomes (roots) which cause problems when managing this species. Small pieces of rhizomes are capable of rejuvenating the plant. The rhizomes also allow the plant to survive over winter when the over ground conspicuous leafy part of the plant dies back to a brown wasted stem.
The leaves are shield or heart shaped usually with a pale stripe down the middle. Flowers are creamy and arise from the tips of stems.
Impact
F. japonica is a threat in open and riparian areas where it speads rapidly to form dense stands, excluding native vegetation and prohibiting regeneration. This reduces species diversity and alters habitat for wildlife. Once stands become established, they are extremely persistent and difficult to remove.
Japanese Knotweed is also of concern to developers and private citizens. This plant has the ability to grow through tarmac and concrete (in some cases within dwellings) and therefore must be cleared completely before starting to build or lay roads.
Where is it from
A native of Japan, Korea, Taiwan and China where both male and female plants are known. This species is now widespread in continental Europe and Britain but only female plants have been recorded to date.
Where are they in Ireland
Japanese Knotweed is very common right across Ireland. It occurs in numerous different types of habitats from road sides to river corridors to waste ground in urban areas.
How did it get here
The date of first introduction to Ireland is not know for certain. It is believed that this plant arrived in the mid to late 1800’s. Regardless of the date of introduction, this plant has spread from gardens into the environment and is now a pest species.
How are they spreading in Ireland
This species is spread by human influence and movement of knotweed plant material or contaminated soil. Movement of road or construction vehicles with contaminated soil either as cargo or as debris on the vehicle can also introduce this species to new areas.
What you can do
Report all sightings.
Download Best Practice Guidance Document to help you tackle this species.
Download the How to Manage Japanese Knotweed at Home leaflet.
Additional reading
Habitas Invasive Alien Species in Northern Ireland
